Scientists Turn to Silk Strands for Surgery, Circuits

Synthetic silk could be the next big thing in biotechnology.

To materials scientists and fashion designers alike, silk is a highly desirable paradox. Its smooth, soft, shimmery folds have made it a mark of luxury since ancient times. Silk is flexible and elastic, and yet it is extraordinarily strong—the strongest natural material known. In fact, its strength rivals that of Kevlar, says Fiorenzo Omenetto, a biomedical engineer at Tufts University.

Silk also manages to be both durable—able to persist for long periods of time once it's been spun—and entirely biodegradable. That quality has made scientists particularly excited about the possibility of using silk in medicine. Taking advantage of advances in materials science and nanotechnology, a number of companies and researchers have begun to furiously develop silk-based medical technologies, including brain implants, optical devices, cell scaffolding and adhesive gels. "The interest has really, really been growing," says Cheryl Hayashi, an evolutionary biologist at the University of California–Riverside who studies spider silk. "I regularly get requests [for silk] from engineers."

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Omenetto and a team from Tufts University and the University of Illinois at Urbana-Champaign created their own silk, boiling silkworm cocoons, extracting the proteins, and then using them to make a thin silk film. In a 2009 paper, published in Applied Physics Letters, the researchers showed that an array of six tiny silicon transistors could be mounted on a square centimeter of this film. Such a device would have significant benefits over current implantable electronics. The silk base becomes flexible when wet, allowing surgeons to carefully mold the implant to the relevant nerves or tissues. Plus, the silk proteins don't provoke an immune response, so the material is readily accepted by the body. Over time, the silk will slowly dissolve, leaving just the electronics behind. "The body reabsorbs the silk," Omenetto says. "It beautifully reintegrates into the body."

This silk-silicon circuit is just one of many potential applications. Some of Omenetto's colleagues at Tufts, as well as scientists at the Legacy Clinical Research & Technology Center in Portland, Ore., have created silk-based brain implants that contain adenosine, a compound that can suppress seizures. In papers published in 2008 and 2009, the researchers demonstrated that the device could reduce epileptic seizures in rats. A company called Oxford Biomaterials is exploring the possibility of using silk to transfer materials to repair bone, cartilage and nerves into the body. Others are creating new kinds of silk-based sutures and adhesive gels.

One reason silk can be used for so many different applications is because so many different kinds of silk exist in nature. If researchers want a dry silk that will withstand high temperatures, for example, they can study spiders that live in the desert; if they want a microbe-, mold- and mildew-resistant one, on the other hand, they could turn to spiders that live in the tropical rainforest. "We can draw on this vast amount of evolution that's gone on in different environments," Hayashi says. "It's like having this big catalogue—you can pick and choose the right spider and the right silk for your application."

The biggest challenge to date has been figuring out how to recreate specific silks in a lab and on a large scale. Researchers have had some success creating spider silk transgenically: The Canadian company Nexia Biotechnologies was able to coax silk fibers out of milk produced by genetically modified goats. Still, because spider silk is made of large and repetitive proteins, the material has proven tough to manufacture, says Sarah Weisman, a chemist in the biomaterials group at Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO).

Weisman and her colleagues at CSIRO have discovered a potential alternative. "We did this wide survey of insect silks and found that a number of them didn't have the disadvantages of spider silk," she says. In particular, the scientists focused on bee silk, which is composed of less complex and unwieldy proteins. The team inserted honeybee silk genes into E. coli bacteria. The bacteria produced silk proteins, from which the researchers extracted silk fibers as strong as those found in nature. The team is still perfecting the process, but once they've nailed the basic fiber, they plan to start modifying the material. "You can add metals and make fibers conductive," Weisman says. "You can add dyes that make them reactive to specific stimuli in the environment."

Tufts' Omenetto and his team have already shown that biological agents, such as enzymes, can be incorporated into silk material. The feat raises tantalizing possibilities. Researchers could add substances that change color to give warning when they're exposed to certain microbes, for example. Surgeons could implant a silk sensor during operations, leaving it to monitor for signs of infection for several months before gradually dissolving into the body. The potential, experts say, is nearly limitless. "Anything that we try with silk seems to work," Omenetto says. "It's remarkable."